(a) Field of the Invention
The present invention relates to a soft austenitic, Co-containing stainless steel alloy having a high resistance to high intensity cavitation making it particularly useful for the manufacture and/or repair of hydraulic machine components.
The invention also relates to hydraulic machine components made of, or covered with such an alloy.
(b) Brief Description of the Prior Art
The cavitation phenomenon to which hydraulic machines such as turbines, pumps, propellers, valves or exchangers are subjected, is a problem well known to specialists. By cavitation phenomenon is to be understood the phenomenon whereby a cavity or a vapor bubble develops in a liquid when the local pressure falls below the vapor pressure. When the pressure rises again above that of the vapor, the gas or vapor bubble abruptly collapses. This implosion is accompanied by powerful physical phenomena, namely by a microjet which follows the bubble and of which the speed may reach several hundred meters per second.
When such a microjet meets a wall, its kinetic energy is changed into a localized shock wave capable of deforming the hardest metallic surface and thus producing an important mechanical erosion. The intensity of the localized stresses produced by such impulses may spread over a very wide range depending on the natural conditions of the liquid, of the temperature and of the presence of foreign gases, of the rate in the pressure variation and of the liquid flow speed. These repeated shocks erode the metallic surface by propagating fatigue fissures (elastic deformation) or by plastic deformation leading to stripping of particles of small dimension.
The observations of damages on numerous groups of hydraulic machine components and the results of accelerated ultrasonic cavitation tests carried out by specialists including the instant inventor, have shown that the hydraulic machine components, especially the hydraulic turbines components, are generally subjected to a broad range of cavitation intensities which can be divided in two categories calling for two different solutions, one of these solutions being applicable to low intensity cavitation, the other one to high intensity cavitation.
Low intensity cavitation in hydraulic machines, especially hydraulic turbines has proved to occur on large areas and to attack mostly carbon steel, leaving stainless steel unaffected. This cavitation produces a slow erosion of carbon steel, which erosion is accelerated by corrosion and galvanic coupling with noble alloys such as stainless steel.
To overcome this part of the problem, the best solution consists in using components entirely made of stainless steel. Another solution consists in covering all the areas of carbon steel components subjected to low intensity cavitation, with a plurality of stainless steel welded overlays, in order to avoid the synergetic effect of cavitation-erosion galvanic corrosion.
On the other hand, high intensity cavitation has proved to occur in hydraulic machine components or groups operating at higher head and water velocities, on small localized areas only, such as, for examples, part of the back of turbine blades. This cavitation produces fast erosion even on high resistance materials such as austenitic stainless steels, at rates between 0.1 and 10 mm per year.
To overcome that part of the problem, high resistance materials are required. Hard alloys such as Co-based, STELLITE*-1 or -6 alloys, aluminum bronze, or highly resilient polymeric materials such as NYLON* 66 have been successfully tried and are used in some particular applications. However, it should be noted that these particular applications are rather limited in practice, because most of known, high resistance materials are difficult to grind and apply in addition of being expensive. FNT * trade-marks
It has recently been found that for some alloys, high hardness is not necessary for high cavitation resistance. K. C. Anthony et al. in their paper &lt;&lt;The Effect of Composition and Microstructure on Cavitation Erosion Resistance&gt;&gt;, 5th Int. Conf. on Erosion by Solid and Liquid Impact, paper 67, Cambridge, England, Sept. 1979, have shown that in Co-based Stellite-alloys, the cavitation-erosion resistance is not affected by lowering the carbon concentration from 1.3 down to 0.3 with the hardness going from 40 to 25 RC. This surprising result which is very important for cavitation erosion damage repair where grinding operation is difficult, has led to extensively test soft, Co-based alloys with a low carbon content, such as STELLITE-21, for the repair of cavitation damages in hydraulic turbines. These tests have proved that the soft, Co-based alloys are much more efficient than the austenitic stainless steel 308 or 301 as repair weld overlay material for localized high intensity cavitation erosion. More particularly, the tested alloys have proved to be easier to grind and, although of a higher cost, more economic to use because they last more than 10 times longer than stainless steels, thereby saving many repair outages.
The above mentioned fact that soft alloys, especially soft Co-based alloys, may have a high cavitation resistance, has not been satisfactory explained yet. Originally, the superior erosion behaviour of cobalt alloys such as STELLITE-6, was attributed to a strain induced martensitic transformation absorbing a significant fraction of the incident cavitation energy. However, subsequent experiments have shown that the contribution of such a martensitic transformation to erosion resistance, if any, is minor (see, for example, D. A. Woodford, &lt;&lt;Cavitation-Erosion-Induced Phase Transformation in Alloys&gt;&gt;, Met. Trans. Vol. 3, p. 1137, May 1972, and S. Vaidya et al, &lt;&lt;The Role of Twinning in the Cavitation Erosion of Cobalt Single Crystals&gt;&gt;, Met. Trans. A, Vol. llA, p. 1139, July 1980). The same experiments have rather shown that any improvement in erosion properties parallels a decrease in stacking fault energy (STE). It has therefore been suggested that the planar slip mode in low SFE materials delays the development of localized stresses required to initiate fracture, thus improving high-cycle fatigue strength.
S. Vaidya et al. have also suggested in their above mentioned article that fine-scale twinning is responsible for the superior erosion resistance of cobalt in hexagonal close pack form (hereinafter referred to as H.C.P. or .epsilon.-phase), this particular form being a low temperature, stable form of cobalt coming from an allotropic transformation occuring at 420.degree. C. in pure cobalt originally in face centered cubic form (such a form being hereinafter referred to as F.C.C. or .gamma.-phase).